Islamic Calendar (thing)

The Islamic Calendar, used by Muslims, is based on the moon. It has 12 months of either 29 or 30 days. Eleven times, every 30 years, an extra day is added to keep the calendar in time with the moon, not the seasons.

Since the Islamic year is just 354 or 355 days long, holidays move backward through the seasons, and each year, a holiday comes eleven days or sooner. In 33 years, the holiday is back where it started.